A network may use both optical fiber and coaxial cable (“coax”) for respective links. For example, the portions of the network that use optical fiber may be implemented using the Ethernet Passive Optical Networks (EPON) protocol, and the EPON protocol may be extended over coaxial cable plants. EPON over coax is called EPOC. The coax physical layer (PHY) used for EPOC communications may be an orthogonal frequency-division multiplexing (OFDM) PHY. Examples of the number of tones that may be used by coax OFDM PHYs include 4096, 8192, or 16384 (e.g., as provided by correspondingly sized FFTs).
Coax signals (e.g., EPOC signals) may be impaired by carrier frequency offset (CFO), which results from a difference between the frequency of a local oscillator in a transmitter and the frequency of a corresponding local oscillator in a receiver. (Other sources of signal impairment include I/Q mismatch and channel distortion.) To reduce costs, it is desirable to use relatively low-accuracy oscillators (e.g., crystal oscillators) in the transmitter and receiver. For example, the oscillators may have an accuracy of 20 parts per million (ppm). The combination of low-accuracy oscillators, the potentially high carrier frequencies anticipated for EPOC communications (e.g., 1 GHz or more), the potentially large bandwidths anticipated for EPOC communications (e.g., up to 100 MHz or more), and the large number of tones anticipated for EPOC communications (e.g., 4K, 8K, or 16K tones) presents a significant challenge with regard to carrier frequency offset. For example, a 20 ppm oscillator with a 1 GHz carrier frequency has a potential frequency error of 20 kHz, resulting in a worst-case frequency offset of 40 kHz between the transmitter and receiver. Assuming a bandwidth of 150 MHz and a coax PHY with 4096 tones (e.g., as provided by a 4096 FFT), the tone spacing is approximately 37 kHz. In this example, the worst-case frequency offset is higher than the tone spacing, which would impair communications. Even if higher accuracy oscillators are used, narrow tone spacing can still exacerbate the effects of carrier frequency offset.
A receiver can correct for frequency offsets by using training fields included in packet preambles. However, training fields consume bandwidth. Given the large number of tones available for coax OFDM PHYs, the data for an entire packet may fit in a few OFDM symbols or even a single OFDM symbol. Therefore, it is not practical to dedicate one or more OFDM symbols as training fields during data communications. Accordingly, there is a need for methods and systems that make efficient use of training fields.
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